Vitamin D in Depression and Suicidality Grudet, Cécile

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Vitamin D in Depression and Suicidality

Grudet, Cécile

2022

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Grudet, C. (2022). Vitamin D in Depression and Suicidality. [Doctoral Thesis (compilation), Department of Clinical Sciences, Lund]. Lund University, Faculty of Medicine.

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Vitamin D in Depression and Suicidality

CÉCILE GRUDET

DEPARTMENT OF CLINICAL SCIENCES, LUND | FACULTY OF MEDICINE | LUND UNIVERSITY

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Vitamin D in Depression

and Suicidality

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Vitamin D in Depression and Suicidality

Cécile Grudet

DOCTORAL DISSERTATION

by due permission of the Faculty of Medicine, Lund University, Sweden.

To be defended at conference room 12, Baravägen 1, Lund. Date 23^rd of September 2022 at 1 pm.

Faculty opponent

Docent Ursula Werneke, Umeå University

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Organization LUND UNIVERSITY

Document name

DOCTORAL DISSERTATION Faculty of Medicine, Department of Clinical

Science Lund Date of issue: 2022-09-23

Author(s) Cécile Grudet Sponsoring organization Title: Vitamin D in Depression and Suicidality

Abstract

Background and aims

The aim of this thesis is to explore the relationship between vitamin D and different aspects of depressive disorders and suicidality, as well as the relationship between vitamin D and inflammation in these conditions.

Material and methods

Study I: Patients (n=59), with diverse psychiatric diagnoses, were enrolled in the study after a suicide attempt.

Seventeen non-suicidal patients with MDD and 14 healthy controls were also included. Interleukin (IL)-1β, IL-6, Tumor Necrosis Factor alpha (TNF)-α, vitamin D2 (25(OH)D2), and vitamin D3 (25(OH)D3) were analyzed.

Study II: Un-medicated, somatically healthy MDD subjects (n=48) and healthy controls (n=54) were enrolled in the study. IL-6, TNF-α, Neutrophil-to-Lymphocyte ratio (NLR), White Blood Cell count (WBC), vitamin D2, and D3 were analyzed.

Study III-IV: Patients with difficult-to-treat depression (Study III: n=202, Study IV: n=263), and healthy controls (Study III: n=46, Study IV: n=51) were recruited. Levels of vitamin D2 and D3 (Studies III and IV), and the inflammatory markers IL-6, TNF-α, CRP, IFN-gamma, IL-10, IL-8, IL-13, and IL-2 were analyzed (Study IV).

Results

Suicidal patients had significantly lower vitamin D levels than both non-suicidal depressed patients and healthy controls (Study I). There was a significant negative association between vitamin D and inflammatory markers in depressed patients, but not in controls (Studies I and II). Patients with ‘difficult-to-treat’ depression had significantly lower vitamin D levels than healthy controls, but vitamin D was not associated with any specific diagnosis or suicidality (Study III). Patients with difficult-to-treat depression had significantly higher IL-6 and IL-8 levels compared to healthy controls (Study IV), and an inflammatory depression subgroup was associated with more severe symptoms of sleeping problems, appetite disturbance, tiredness, and anhedonia as well as low vitamin D in combination with high IL6 and IL-8 levels.

Conclusion

Our findings are consistent with previous studies of lower vitamin D levels in depression, which might play a role in the pathophysiology of the disorder. Low vitamin D and chronic low-grade inflammation may be part of the same subtype of depression, with a distinct symptom profile related to inflammation, but more research is needed before this can be established.

Key words Vitamin D, major depressive disorder, suicidality, inflammation Classification system and/or index terms (if any)

Supplementary bibliographical information Language English

ISSN 1652-8220 ISBN 978-91-8021-285-4

Recipient’s notes Number of pages 93 Price

Security classification

I, the undersigned, being the copyright owner of the abstract of the above-mentioned dissertation, hereby grant to all reference sources permission to publish and disseminate the abstract of the above-mentioned dissertation.

Signature Date 2022-08-19

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Vitamin D in Depression and Suicidality

Cécile Grudet

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Cover photo by Photographer @ jroberpg Copyright pp 1-93 Cécile Grudet

Paper 1 © Psychoneuroendocrinology (by Elsevier Ltd), OA Paper 2 © Journal of Affective Disorder (by Elsevier Ltd)

Paper 3 © Comprehensive Psychoneuroendocrinology (by Elsevier Ltd), OA Paper 4 © by the Authors (unpublished Manuscript)

Faculty of Medicine Department of Psychiatry

ISBN: 978-91-8021-285-4 ISSN: 1652-8220

Printed in Sweden by Media-Tryck, Lund University Lund 2022

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I dedicate this thesis to all people who face the challenges of long-lasting depressive symptoms in their lives.

You are strong and brave and deserve the best of support

anyone can get!

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Abstract

Background and aims

The aim of this thesis is to explore the relationship between vitamin D and different aspects of depressive disorders and suicidality, as well as the relationship between vitamin D and inflammation in these conditions.

Material and methods

Study I: Patients (n=59), with diverse psychiatric diagnoses, were enrolled in the study after a suicide attempt. Seventeen non-suicidal patients with MDD and 14 healthy controls were also included. Interleukin (IL)-1β, IL-6, Tumor Necrosis Factor alpha (TNF)-α, vitamin D2 (25(OH)D2), and vitamin D3 (25(OH)D3) were analyzed.

Study II: Un-medicated, somatically healthy MDD subjects (n=48) and healthy controls (n=54) were enrolled in the study. IL-6, TNF-α, Neutrophil-to-Lymphocyte ratio (NLR), White Blood Cell count (WBC), vitamin D2, and D3 were analyzed.

Study III-IV: Patients with difficult-to-treat depression (n=202 Study III, n=263 Study IV), and 46 (Study III) and 51 (Study IV) controls were recruited. Levels of vitamin D2 and D3 (Studies III and IV), and the inflammatory markers IL-6, TNF- α, CRP, IFN-gamma, IL-10, IL-8, IL-13, and IL-2 were analyzed (Study IV).

Results

Suicidal patients had significantly lower vitamin D levels than both non-suicidal depressed patients and healthy controls (Study I). There was a significant negative association between vitamin D and inflammatory markers in depressed patients, but not in controls (Studies I and II). Patients with ‘difficult-to-treat’ depression had significantly lower vitamin D levels than healthy controls, but vitamin D was not associated with any specific diagnosis or suicidality (Study III). Patients with difficult-to-treat depression had significantly higher IL-6 and IL-8 levels compared to healthy controls (Study IV). An inflammatory depression subgroup was associated with more severe symptoms of sleeping problems, appetite disturbance, tiredness, and anhedonia as well as low vitamin D in combination with high IL6 and IL-8 levels.

Conclusion

Our findings are consistent with previous studies of lower vitamin D levels in depression, which might play a role in the pathophysiology of the disorder. Low vitamin D and chronic low-grade inflammation may be part of the same subtype of depression with a distinct symptom profile related to inflammation, but more research is needed before this can be established.

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List of papers

The thesis is based on the following papers, which will be referred to in the text by their Roman numerals.

Study I

Grudet, C., Malm, J., Westrin, A., & Brundin, L. (2014). Suicidal patients are deficient in vitamin D, associated with a pro-inflammatory status in the blood.

Psychoneuroendocrinology, 50, 210-219. doi:10.1016/j.psyneuen.2014.08.016 Study II

Grudet, C., Wolkowitz, O. M., Mellon, S. H., Malm, J., Reus, V. I., Brundin, L., Nier, B. M., Dhabhar, F. S., Hough, C. M., Westrin Å., Lindqvist, D. (2020).

Vitamin D and inflammation in major depressive disorder. J Affect Disord, 267, 33-41. doi:10.1016/j.jad.2020.01.168

Study III

Grudet, C., Lindqvist, D., Malm, J., Westrin, Å., & Ventorp, F. (2022). 25(OH)D levels are decreased in patients with difficult-to-treat depression. Comprehensive Psychoneuroendocrinology, 10, 100126. doi:10.1016/j.cpnec.2022.100126 Study IV – Manuscript (to be submitted shortly)

Suneson, K., Grudet, C., Ventorp, F., Malm, J., Asp, M., Westrin, Å., Lindqvist, D. An inflamed subtype of difficult-to-treat depression.

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Preface

I became interested in vitamin D a long time ago due to my own difficulties enduring the long, dark winters in Sweden. Thus, since I suffer from seasonal affective disorder (SAD), I am naturally curious about everything that could be associated with a depressed mood during the winter, or with a depressed mood in general.

Therefore, in 2012, I attended a seminar about affective disorders where (my upcoming co-supervisor) associate Professor Lena Brundin discussed the possible connection between inflammation and depression, and what she said made much sense to me. Shortly after that, Professor Johan Malm (also one of my upcoming co- supervisors) held a class at medical school on the extra-skeletal effects of vitamin D in the body, highlighting vitamin D’s connection to the immune system. Like most people, I had heard that vitamin D could be related to depression in some way, and instantly, during Johan Malms’ class, it became obvious to me that it could be the effect of vitamin D on the immune system that linked vitamin D to depression!

Curious as I am, I was eager to explore my hypothesis, so I turned to professor Åsa Westrin (my upcoming, first, and main supervisor), whom I knew was involved in suicide research in Lund, and asked her if I could do “summer research” on vitamin D under her supervision. She generously took me under her wings and accepted me as a summer researcher and ultimately as a PhD student in 2015.

The road leading up to this thesis has been bumpy, but nevertheless focused on the possible connection between vitamin D and depression. At the beginning of my PhD studies, my somewhat naïve enthusiasm for doing research, combined with an attitude of unlimited possibilities, led me to wanting to explore every single idea I had regarding the relationship between vitamin D and psychiatric illness. Certainly, quite a few studies were discarded along the way, some of which only got to the point of a preliminary title, such as “Is there an association between vitamin D levels and 5-HIAA cerebrospinal fluid levels in psychiatric patients with the genetic makeup SS or SL alleles of the promoter region (5-HTTLPR) of the serotonin uptake transporter (5-HTT)?”, or “Is mental fatigue in depressed patients associated with vitamin D levels?”. Another discarded study, which I put significant work and effort into, was titled ‘Is vitamin D levels in umbilical cord associated with suicide later in life?’. After giving me extensive freedom in my work, Åsa finally stepped in and guided me in my thesis so it became narrowed to focus solely on the relationship between vitamin D and inflammation in depression and suicidality. For this, I am very grateful.

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Populärvetenskaplig sammanfattning

Bakgrund

Depression är en sjukdom som är så vanlig i Sverige idag att den klassas som en folksjukdom. Folkhälsomyndigheten har uppskattat att mer än var tredje kvinna och nästan var fjärde man kommer att uppleva en eller flera depression(er) under sin livstid. Depression orsakar stort lidande för den enskilde individen och påverkar också ofta även närstående. Att ha en psykisk sjukdom innebär även en stor riskfaktor för självmord och av alla psykiska sjukdomar sticker depression ut som den med störst risk för självmord. Trots att depression är en så pass vanlig sjukdom är dess bakomliggande orsaker inte helt kända. Man tror att orsaken består av en kombination av genetiska, biologiska, miljömässiga och psykologiska faktorer, som alla bidrar till en persons individuella sårbarhet för att utveckla depression.

Sjukdomens komplexitet gör den svår att studera och det är inte troligt att en enskild förklaringsmodell kan förklara alla olika typer av depressionssjukdomar. Det är sannolikt så att det finns olika subgrupper inom den breda gruppen ”deprimerade”, med olika orsaker bakom utlösandet av depressionen, och därför är det inte heller troligt att samma typ av behandling kommer att fungera på alla. Idag är den vanligaste behandlingsformen för depression antidepressiva läkemedel. Det är dock en betydande andel deprimerade som inte svarar tillräckligt bra på antidepressiv behandling. Så många som 70 % av de som påbörjar en antidepressiv behandling blir inte helt friska med hjälp av denna, och i många fall kan depressionen övergå i en kronisk form. Behovet av ytterligare behandlingsmöjligheter är därför stort.

Under de senaste decennierna har den forskningsinriktning som försöker förstå de biologiska orsakerna bakom depression utvecklats med nya och förfinade metoder.

Det långsiktiga målet med denna forskning är att bidra till utvecklandet av s. k.

individanpassad vård, med en bättre förståelse för den enskilda individens underliggande sårbarhet och bidragande orsaker till depressionssjukdom. Med en individanpassad vård är förhoppningen att psykiatrin kommer att kunna erbjuda bättre och mer effektiva behandlingsformer i framtiden.

Inom den biologiska forskningen finns en forskningsgren som undersöker om inflammation kan ha ett samband med depressiv sjukdom. Många studier har visat att deprimerade och/eller suicidala individer, på gruppnivå, har högre inflammationsgrad i kroppen än psykiskt friska individer. En hypotes är att detta inte gäller alla typer av depressiv sjukdom, utan att det finns en subgrupp av

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”inflammatorisk depression” med symptom såsom oförmåga att känna, trötthet, utmattning, förändrad aptit och minskat eller ökat sömnbehov. Det är allmänt känt att D-vitamin är viktigt för skeletthälsan, men vad många inte vet är att D-vitamin också har stor påverkan på flera andra system och funktioner i kroppen. En av de viktigaste effekterna D-vitamin har i kroppen, utöver skeletthälsan, är att den påverkar immunförsvaret på ett sätt som man förenklat kan säga är ’anti- inflammatoriskt’. Detta innebär att om du har låga nivåer av D-vitamin så är det möjligt att du har en högre inflammationsgrad i kroppen, vilket också är något som har visats i olika somatiska sjukdomar, som till exempel vissa cancerformer, autoimmuna sjukdomar, diabetes och hjärt- och kärlsjukdomar.

Med tanke på forskningsfältet som talar för att inflammation kan vara en bidragande orsak till depression och att låga D-vitaminnivåer har kopplats ihop med depression i många tidigare studier, vill vi undersöka hur inflammation är kopplat till D-vitamin hos deprimerade individer. Det övergripande syftet med denna avhandling var att undersöka kopplingen mellan D-vitamin och olika aspekter av depressiv sjukdom och suicidalitet, samt kopplingen mellan D-vitamin och inflammation vid dessa tillstånd.

Material och metoder

Alla studierna i avhandlingen var tvärsnittsstudier, dvs de bygger på att studera ett material vid ett enda tillfälle och vi kan därmed inte säga något om orsakssamband.

Sammantaget ingick fyra olika kliniska patientgrupper i avhandlingen; en grupp patienter med olika psykiatriska diagnoser som nyligen gjort ett självmordsförsök (Studie I), två olika grupper med medelsvår-till-svår klinisk depression som inte nyligen hade gjort något självmordsförsök (Studie I; Studie II) och en grupp med svårbehandlad depression (Studie III; Studie IV). I alla studierna ingick också en kontrollgrupp med friska individer. Patientgrupperna i Studie I, III och IV kom främst från Lund med närområde och i Studie II kom deltagarna från San Fransisco, USA. Alla patienter var diagnosticerade enligt The Structured Clinical Interview for Axis-I Disorders (DSM-IV) och noggrant utredda vad gäller klinisk symptombild med bland annat olika självskattningsskalor.

Vitamin D och olika inflammatoriska markörer analyserades och jämfördes mellan studiegrupperna i de respektive studierna.

I studie IV delades patientgruppen in i två grupper baserad på närvaron av låggradig inflammation eller ej.

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Resultat

I alla studier fanns en negativ koppling mellan D-vitamin och inflammation hos deprimerade patienter, dvs, låga D-vitaminnivåer var kopplade till högre inflammationsgrad, vilket inte sågs hos friska kontroller. Patienter med svårbehandlad depression hade lägre D-vitaminnivåer än friska kontroller, men inga skillnader sågs mellan patienter med olika typer av depressionsdiagnoser. Ingen av studierna visade någon koppling mellan D-vitamin och suicidalitet, förutom Studie I, där vi fann en skillnad i D-vitaminnivå mellan patienter som nyligen hade gjort ett suicidförsök och icke-suicidala deprimerade patienter och friska kontroller. I studie IV fann vi att ”inflammatorisk depression” var kopplat till såväl lågt vitamin D som vissa specifika symptom, såsom svårighet att känna känslor, trötthet, högre utmattning, förändrad aptit och sömnstörning.

Slutsatser

Resultaten i avhandlingen visar att det finns en koppling mellan låga nivåer av D- vitamin och depressiva syndrom. Mycket pekar också på att det finns en koppling mellan låga D-vitaminnivåer och ökad inflammation hos patienter med depressiva syndrom. Våra resultat kan tala för att lågt vitamin D och inflammation båda är del av en undergrupp av depression med vissa typer av symptom, men denna hypotes måste undersökas ytterligare innan man kan dra fasta slutsatser.

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Abbreviations

MDD Major depressive disorder

SI Mild-to-moderate suicidal ideation/suicidal ideation

NSI Non-(mild-to-moderate) suicidal ideation/non-suicidal ideation hgSI High-grade suicidal ideation

lgSI Low-grade suicidal ideation

MINI Mini International Neuropsychiatric Interview

SCID-I Structured clinical interview for DSM-IV. Axis I Disorders SCID-II Structured clinical interview for DSM-IV. Axis II personality

disorders

CPRS Comprehensive Psychopathological Rating Scale HDRS Hamilton Depression Rating Scale

MADRS Montgomery-Åsberg Depression Rating Scale SUAS-S Suicide Assessment Scale (self-reported version) YPAS Vigorous Activity Index Score

TNF-α Tumor Necrosis Factor-alpha IL Interleukin

IFN-γ Interferon-gamma CRP C-reactive protein

NLR Neutrophil-to-Lymphocyte Ratio WBC White blood cell count

Th-1 T helper type 1 cell Th-2 T helper type 2 cell VDR Vitamin D receptor UVB Ultraviolet B radiation RCT Randomized controlled trial PCA Principal component analysis

AHDH Attention deficit hyperactivity disorder

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Definitions

Vitamin D

Vitamin D

The sum of the two circulating biologically inert metabolites of vitamin D, i.e., 25(OH)D2 and 25(OH)D3, will be referred to as ‘vitamin D’ in the thesis. These are the metabolites we measure in the blood to assess vitamin D status.

Active vitamin D

The sum of the two active metabolites of vitamin D, i.e., 1,25(OH)2D2 and 1,25(OH)2D3. These metabolites bind to the intracellular vitamin D receptor, predominantly in the nucleus of the target cells.

Deactivated vitamin D’/’inactive vitamin D

Vitamin D is catabolized in the kidneys to both 24,25(OH)2D and 1,24,25(OH)3D.

These two metabolites are biologically inert and excreted into the feces.

Vitamin D concentration in the blood

The mostly used units for measurement of vitamin D concentration in the blood are ng/mL and nmol/L, where 1 ng/mL equals approximately 2.5 nmol/L (2.496). The International System of Units (SI) for vitamin D is nmol/L, therefore, this unit is used in the thesis.

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Psychiatry

Suicidality, suicidal ideation

No “gold standard” definition exists. It refers to a wide range of behaviors with suicidal intent, such as death by suicide and attempts at suicide, as well as suicide- related thinking, such as, “I have no reason to live.”

Suicide attempt

In Study I, we used Beck et al.’s (1972) definition of suicide attempt: “Those situations in which a person has performed an actually or seemingly life-threatening behavior with the intent of jeopardizing his/her life or to give the appearance of such intent, but which has not resulted in death’ (1).

Violent suicide attempt

Suicide attempts are classified as violent or non-violent based on the following criteria, as previously defined by Åsberg et al. (1976) and Paykel and Rassaby (1978): “Non-violent suicide attempts include drug overdoses and single wrists cuts or a combination of these. All other attempts were classified as violent” (e.g., hanging, drowning, suffocating, several deep cuts, poisoning, intentionally throwing themselves into traffic, etc.) (2, 3).

Repeaters

Individuals who made one or more suicide attempts before the index suicide attempt (i.e., the suicide attempt preceding inclusion in the study).

Difficult-to-treat depression

The patients included in Studies III and IV had previously been diagnosed with an affective disorder with insufficient treatment response, according to the outpatient psychiatrist, who was herein defined as having a “difficult-to-treat depression.” An insufficient treatment response” was defined as not having achieved remission with previous and ongoing treatments during the current depressive episode.

Suicidal depression

A depressive episode in which the patient has attempted suicide. Suicidal depression is not a clinical diagnosis.

Inflamed depression

An inflamed depression is defined in Study IV as a depression subtype in which the patient expresses a low-grade inflammation in combination with specific depressive symptoms associated with an inflamed state in the body. There is currently no clinical consensus on how to define “inflamed depression,” although a cut-off level of 3 mg/L on C-reactive protein (CRP) has been used in previous studies (4-8) and is a well-established indicator of chronic low-grade inflammation (9). The depressive symptoms associated with inflamed depression are, as interpreted by us, the inability to feel, lassitude, fatiguability, changes in appetite, and reduced or increased sleep.

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Context of this thesis

Depression is known to have a complex etiology, including genetic, biological, environmental, and psychological factors. Thus, it is not possible that one hypothesis can explain all types of depression, or that one-size-fits-all treatments are a successful approach to treating depression. Most likely, there are subgroups among individuals with depressive symptoms where different combinations of susceptibility factors add up to outline an individual’s proneness to developing a depressive disorder.

During the last decades, the field of biological psychiatry that investigates genetic, and other biological factors in relation to depression and suicidality, have expanded and developed new and more refined methods. The long-term goal of this research field is to better understand individual factors connected to the development, severity, and prognosis of psychiatric illnesses—ultimately leading to personalized treatment for depressive disorders. One branch of this research field investigates the role of inflammation in relation to different aspects of depression and suicidal behavior. Since vitamin D has a profound impact on the immune system (10-12), which can be briefly described as “anti-inflammatory,” and low vitamin D levels have been suggested to be related to depression, it is highly relevant to investigate the relationship between vitamin D and inflammation in depressed and suicidal individuals.

By using clinical samples in this thesis, I investigated the relationship between vitamin D and inflammation in depressed individuals with different levels of suicidality. The main aim was to better understand the role of vitamin D in the development of depressive disorders and whether this role is more pronounced in certain subgroups of depression.

At the very beginning of my thesis work, there were no prior studies, to my knowledge, exploring the relationship between vitamin D and inflammation in depression/suicidality. Nor were there any studies investigating vitamin D in different subgroups of depressive disorders. Even now, at the end of my thesis, this research field is still limited. I hope that my contribution to this field of research will inspire future research to include the biological downstream effects of vitamin D, with the aim of expanding and ameliorating treatment options for depression and/or suicidality.

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Introduction

Major depressive disorder and suicidality

Major depressive disorder (MDD) causes extensive morbidity and mortality worldwide and is a great risk factor for death by suicide. It has been estimated that 40–60% of those who die by suicide suffer from depression (13, 14). Nevertheless, the pathophysiology and somatic manifestations are not yet fully understood, and the complexity of the disease makes it difficult to study.

Today, according to the World Health Organization (WHO), approximately 400 million people are suffering from depression worldwide, and about 700, 000 individuals die by suicide each year (15). In Sweden, it is estimated that more than every third woman, and almost every fourth man, will develop one or more depressive episode(s) during their lifetime (16). According to the National Centre for Suicide Research and Prevention of Mental III-Health (NASP), around 9,000 suicide attempts are made in Sweden each year, and around 1,500 people die by suicide (17). Consequently, depression is classified as a public disease (18), and it causes enormous negative effects on a societal and economic level. Most importantly, though, the life of a depressed person is most often compromised on many levels, with profound suffering and many negative consequences emerging in relation to the illness. In its most severe form, the risk of a suicidal act is also present.

Hence, there is an indispensable need for efficacious treatment options for depression. Sadly, only about 30% of depressed individuals reach full remission after the first line of treatment, and in many cases, depression may develop into a chronic illness (19, 20).

During the last decades, great efforts have been made to better understand the biological underpinnings of depression, with the long-term aim of developing personalized and more effective depression treatments (20-23). Several lines of evidence suggest a link between chronic low-grade inflammation and a subtype of depression (in this thesis, called “inflamed depression”) (24-28). Candidate mechanisms that have been explored in recent years include, but are not limited to, autoimmune disorders, infections, and genetic predisposition (29). Also, since vitamin D has a role in regulating the immune system, a deficiency may be an upstream cause of inflamed depression, but this has not been investigated previously.

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Major depressive disorder

Major depressive disorder (MDD) is a clinically diagnosed mental disorder defined by a set of specific criteria. An MDD diagnosis, according to the Diagnostic and Statistical Manual of mental disorders (DSM) criteria (the classification system used in all studies included in the thesis), is defined by the presence of five or more specific core symptoms lasting for two continuous weeks (most of the day and nearly every day), which also causes significant distress or impairment to the individual (30). These depressive symptoms are depressed mood and/or markedly diminished interest or pleasure in almost all activities, significant weight loss/weight gain or decreased/increased appetite, insomnia or hypersomnia, psychomotor agitation or retardation, fatigue/loss of energy, feelings of worthlessness or excessive inappropriate guilt, diminished ability to think/concentrate or indecisiveness, recurrent thoughts of death (not just fear of dying), recurrent suicidal ideation or suicide attempt, or a specific plan for attempting suicide. Other depressive disorder diagnoses in the DSM are bipolar depression (with episodes of mania), dysthymic disorder (chronic low-grade depression), and depressive disorder not otherwise specified (aspects of depression but do not meet all criteria of an MDD).

Often, an MDD is an illness in which individuals experience more than one episode during a lifetime perspective (20). Findings from several large-scale studies have shown that most patients have recovered, or partially recovered within one year from the depressive episode (31). However, about 80% of patients diagnosed with a single MDD episode will later experience recurrent MDD episodes (32), and in 20–30% of cases, MDD will take a chronic illness course (33). The risk of developing depression is defined by a combination of unfavorable genetic and environmental factors, where genetic factors represent approximately 30–40% of the risk. It is also known that being female, having a family history of mental illness, having adverse childhood experiences (abuse, neglect, violence, etc.), and experiencing more recent stress factors are all risk factors for developing depression (23).

Suicidality

Suicidality refers to a wide range of behaviors with suicidal intent, such as death wishes, suicide thoughts, suicide plans, suicide attempts, and death by suicide.

Suicide-related thinking, for example, “I have no reason to live,” is also included in the concept of suicidality. However, the construct “suicidality” does not have any exact definition and is therefore difficult to study. In accordance with depressive disorder, suicidality includes complex interactions between predisposing factors and environmental factors. Studies on suicide prevention have shown that predicting a suicide attempt/suicide is difficult (14). Nevertheless, several risk factors behind

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suicide attempts or death by suicide have been investigated and identified, both at population-based levels and at individual levels (13). Many risk factors overlap with risk factors for developing depression, which is understandable because having depression (or other mental illnesses, schizophrenia-spectrum disorders, or substance use disorders) augments the risk of death by suicide. Thus, having a psychiatric diagnosis is a major risk factor for death by suicide (14, 34). However, the main risk factor for death by suicide is one or more previous suicide attempt(s) (34, 35). Certain personality traits have also been associated with elevated suicide risk, where impulsivity and aggression stand out in most models of suicide (14, 36).

Contributing environmental risk factors include a lack of social support, economic difficulties, stressful life events, and access to lethal means (13, 34).

As noted before, it is extremely hard to predict a future attempt at suicide (34). To help clinicians, different structured suicide assessment instruments can be used to better understand the patient’s present situation and state of mind (35, 36). One of these is the Suicide Assessment Scale (SUAS) (37), which has been used in some of the studies included in this thesis. However, the SUAS is, in several sites in Sweden, gradually being replaced by the Columbia-Suicide Severity Rating Scale (C-SSRS), suggested to be one of the best tools of its kind (38). It is a short questionnaire designed to work in most settings, research included, by any individuals who have received training in administrating the scale. In research settings, structured suicide assessment instruments are useful when, for example, the degree of suicidality or suicide risk is measured and associated with other studied factors. However, the division of patients into “high suicidal ideation” or

“low suicide ideation” groups will never be perfect, and always be based on the researchers’ choice of assessment instruments and cut-off levels.

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An introduction to vitamin D

History

The history of vitamin D goes far back in time. The first substantial description of the disease “rickets” is dated as early as 1645 in a book written by Wistler, a medical doctor from England (39). Similar disorders had been described previously for centuries, but Wistler provided the very first, printed, clear description of a disease that caused poor mineralization and deformations of the skeleton, naming it “the rickets”—a disease we now know is caused by vitamin D deficiency. The disease became an epidemic during the 19^th century, especially in England and Northern Europe, due to urbanization and deprived sunlight. The incidence among children in England was as high as 70–80% at the time, and this new disease was therefore called “English Disease” (40).

During the late 19^th century and early 20^th century, many biologic scientists were trying to understand the indispensable components of diets, and in 1913, the existence of vitamins was established by McCollum and Davis at the University of Wisconsin (41). In working with rat experiments, they discovered that cod-liver oil, together with butterfat, contained a water-soluble micronutrient that they called

“vitamin A” and that this diet was able to cure an eye disease, xerophthalmia. Some years later, Sir Edward Mellanby, inspired by McCollum’s work, was able to produce rickets in dogs, which he kept indoors for practical reasons and thus deprived of sunlight, and cured them with a cod-liver oil treatment. Mellanby was not aware that cod-liver oil also contained vitamin D; hence, he speculated that curing rickets was only another property of vitamin A.

McCullom, now aware of Mellanby’s work, started to question whether it really was vitamin A in cod-liver oil that cured rickets. He managed to destroy the vitamin A activity in the cod-liver oil and discovered hereby that cod-liver oil could no longer cure xerophthalmia but was still able to cure rickets. McCollum then understood that cod-liver oil also contained another micronutrient, which he named “vitamin D” (40).

Today, vitamin D deficiency is a major global public health issue. It is estimated that 1 billion people are deficient worldwide, and that 50% of the population has insufficient levels (see Figure 2) (42). Nutritional rickets, which can easily be fully prevented by vitamin D supplements, continue to be a global health issue in Western countries.

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Glisson examines a child with rickets as the mother looks on. Two more children with rickets play in the background, and bones deformed by rickets are hand on the wall. Glisson was a physician who, along with Wistler, was one of the first to clearly describe the disease (1650). Copyright: Free to use from the US National Library of Medicine digital collection.

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Metabolism

Although once named vitamin D, it is not actually a vitamin but rather a fat-soluble steroid hormone, which comes in six forms and undergoes several metabolizing steps to form its active metabolite, 1α, 25-dihydroxyvitamin D (1,25(OH)2D), also known as calcitriol (43) (see Figure 1). There are two forms of vitamin D: vitamin D2 and vitamin D3. Both vitamin D2 and vitamin D3 can be obtained through exogenous sources, such as varied diets and, in more significant amounts, vitamin D supplements and fortified food. Vitamin D3 can also be produced in the skin.

Metabolites from exogenous sources enter the bloodstream after intestinal absorption. However, the main source of vitamin D in our body is produced endogenously when the skin is exposed to ultraviolet B (UVB) radiation (wavelength 280-315 nm), and UVB rays penetrate the skin (44). Since only vitamin D3 can be synthesized via the skin, and vitamin D3 is the most important source of vitamin D in the body, I will focus solely on describing the endogenous vitamin D3

metabolicpathway.

As previously mentioned, endogenous synthetization starts when UVB rays penetrate the skin, subsequently initiating a two-step reaction. First, there is a conversion of 7-dihydrocholesterol (7-DHC) in the skin, followed by thermal isomerization, which generates the end-product pre-vitamin D3 (cholecalciferol).

After entering the bloodstream, pre-vitamin D is rapidly hydroxylated mainly in the liver by 25-hydroxylase (CYP27A1) to form vitamin D (25[OH]D), also known as calcidiol, which is the biologically inert, circulating form of vitamin D. Second, and vitamin D-activating, conversion occurs in the kidneys by 1α-hydroxylase (CYP27B1), where most of the active form of vitamin D is synthesized, i.e., 1,25(OH)2D (calcitriol) (42, 45). Both vitamin D and active vitamin D are predominantly bound to vitamin D-binding protein (DBP) when circulating in the blood. Notably, although the kidneys are the main source of circulating active vitamin D, the vitamin D-activating enzyme is present in numerous other cells and tissues as well, among which, cells related to the immune system, a fact of great importance to this thesis. Thus, by the extrarenal presence of the vitamin D- activating enzyme, these extrarenal sites can supply local needs for active vitamin D in a paracrine/autocrine manner, which will be described further in “mechanisms of action” (46) (see Figure 1).

Activated vitamin D is a potent hormone involved in the crucial regulation of blood calcium and phosphate levels in extracellular fluids by affecting intestinal absorption, renal excretion, and bone calcium turnover (43, 47). Since excessive calcium in the blood is potentially very harmful to the body by, for example, interfering with heart and brain function, vitamin D activation triggered by low calcium levels is under strict regulation. Calcium and phosphorus homeostasis, and the maintenance of bone substance, is thus regulated by a tightly controlled feedback loop between parathyroid hormone (PTH) and vitamin D, which is sometimes called

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the “calcium-vitamin D-PTH axis.” PTH is stimulated and secreted upon a decrease in calcium levels in the blood, which in turn activates the synthesis of active vitamin D in the kidneys. Active vitamin D subsequently stimulates an increase in intestinal absorption of calcium and phosphorus. A continued hypocalcemia and exposure to increased PTH levels may also lead to calcium and phosphorus resorption from bone, as well as renal and intestinal calcium absorption, mediated by both PTH and active vitamin D. However, in the case of a rise in blood calcium levels, PTH secretion drops, consequently leading to decreased renal synthesizing of active vitamin D and decreased calcium mobilization.

As previously mentioned, active vitamin D can be catabolized in extrarenal tissues, generated by the expression of 24-hydroxylase. This is an important regulation mechanism of auto or paracrine signals originating from locally produced active vitamin D. The amount of active vitamin D synthesis and degradation in extrarenal tissues is under the control of local factors, such as cytokines and growth factors.

However, the mechanisms behind this process are incompletely understood (48).

The self-regulation of UVB-induced vitamin D production is generated by sun exposure (“tanning”), which in turn leads to an increase in epidermal melanin content. As melanin competes with 7-DHC to absorb UVB photons, the possibility of 7-DHC conversion to pre-vitamin D in the skin decreases. Adding to less availability of pre-vitamin D in the body, and thus less vitamin D originating from the skin, the associated photo-isomerization of excess pre-vitamin D (and vitamin D) into its inactive metabolites also occurs (44).

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‘

Figure 1. Vitamin D metabolism

Mechanisms of action

Since both vitamin D and active vitamin D are lipophilic molecules, they must be transported in the circulation bound to plasma proteins, predominantly DBP. After a release from DBP, vitamin D binds to intracellular vitamin D receptors (VDR), which are members of the superfamily of nuclear receptors for steroid hormones and are principally located in the nucleus of target cells. Through vitamin D responsive elements (VDREs), vitamin D acts as a transcription factor and alters the expression of vitamin D responsive genes by significantly enhancing or suppressing the rate of the gene transcription process (48). VDRs are present in almost all tissues and cell types and are thus located in most organs, for example, in the heart, skin, gonads, brain, prostate, and breasts. A genome-wide mapping of vitamin D receptor binding, conducted by Ramagopalan et al., showed that stimulation with active vitamin D regulates the expression of 229 genes directly, and more than 2,000 genes indirectly (49).

However, the genes affected by active vitamin D also have multiple functions beyond regulating calcium homeostasis in the body. These genes have been shown to be involved in, for example, cellular proliferation, differentiation, apoptosis,

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angiogenesis, and modulation of the immune system (10, 11). Due to the presence of both VDR and the vitamin D-activating enzyme in numerous tissues and cells that are not associated with skeletal health, vitamin D can act in autocrine/paracrine ways, independently from the activation step occurring in the kidneys, and this will be of great importance to this thesis.

Vitamin D, like other steroid hormones, is also able to generate responses that are too rapid to include changes in gene expression. It is suggested that these rapid responses are mediated by cell surface receptors on the cell membrane and/or cytoplasm, where VDR is a candidate receptor, since several studies imply that the rapid actions of vitamin D require the presence of VDR as a mediator (48). The function of non-genomic actions remains uncertain in most cells; however, many studies have shown that non-genomic effects do not seem to be critical for vitamin D-mediated gene activation (49, 50). These non-genomic effects include, for example, increases in intracellular calcium levels and stimulation of intestinal calcium transport (51).

Vitamin D deficiency

Vitamin D deficiency and insufficiency are of pandemic proportions, with approximately 50% of the global population being deficient or having insufficient levels (42, 52, 53). However, it must be said that data about vitamin D levels are missing from numerous countries in the world, especially from countries in South America and Africa. Nevertheless, in a recent systemic review of vitamin D status worldwide, Palacios et al. (2015) concluded that low vitamin D status is a global issue in all ages, highlighting the problem to be most pronounced in the Middle East, particularly in girls and women (see Figure 2) (54). Even in countries with access to sun exposure all year round, deficiency has been shown to be common. In Australia, for example, 31% of the adult population were deficient (<50 nmol/L), and 73% had insufficient vitamin D levels (<75 nmol/L) (55). As for the situation in Sweden, large population-based studies are currently lacking. However, there is a reasonably up-to-date study from Gothenburg, Sweden (2017) examining seasonal variations in vitamin D levels in healthy individuals by measuring vitamin D levels of 40–60 participants every month for one year. In total, 540 blood donors were included in the study. They found that the mean serum levels over the year were insufficient: 60 nmol/L (ranging from 47 nmol/L in February to 82 nmol/L in July), and about 50% of the participants were vitamin D deficient (<50 nmol/L) during the winter months (56).

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Several factors contribute to the vast global deficiency, many of which are connected to our modern lifestyle with subsequent sun-avoiding behavior, which in turn impedes vitamin D synthesis (57). The knowledge of the connection between sun exposure and skin cancer has increased the use of sunscreen, sun- avoidant behavior, and the use of sun-protecting clothing. Other sun-related factors are industrialism, which has largely changed the working environment from outdoors to indoors, and the younger generation spending more and more time indoors, consequently at the expense of less outdoor physical activity. Also, there are several environmental factors hindering year-round vitamin D synthesis, such as latitude, season, and amount of air pollution. An important factor in northern countries is the high latitude (Lund, Sweden, 55.7°N), since the UVB availability above and below approximately 33° does not result in any significant vitamin D production during winter time (58). A study modeling UVB availability in nine European countries/regions, with latitudes ranging from 35 to 69°N, showed that all countries/regions had significant seasonality in UVB availability.

However, the number of months in which UVB availability was too low for significant skin synthesis was 7-8 months in northern Europe, in contrast to being basically absent in the very south of Europe (59, 60). Lastly, personal

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characteristics also affect a person’s ability to produce vitamin D from sun exposure, i.e., skin pigmentation and age. Some groups of individuals have a particularly high risk of vitamin D deficiency for various reasons. One is the different genetic variants in deleterious alleles at three loci, confirmed by Wang et al., which more than doubled the odds of having insufficient vitamin D levels (<75 nmol/L) (61). Other individuals who are at higher risk for having low vitamin D levels are pregnant women, people with increased skin pigmentation, people with certain medical conditions, and obese individuals (see Figure 3). Therefore, an obese, pregnant, dark-skinned woman from a culture that includes whole-body clothing, who has a genetic variant susceptible to insufficient vitamin D levels, and who lives in a Nordic country, is most likely severely deficient in vitamin D.

A 2016 study from Sweden indeed showed that 73% of 114 Somali women of reproductive age living in Sweden were severely deficient in vitamin D (<25 nmol/L) and none had levels above 75 nmol/L, which is considered at sufficient levels. Only 5% had vitamin D levels above 50 nmol/L (62).

It is essential to recognize that the definitions of vitamin D deficiency/sufficiency are, and have been under vivid debate for a long time, and there is no consensus yet regarding which levels are to be considered sufficient for optimal health (59).

Most public nutrition guidelines emerge from thresholds primarily related to musculoskeletal health, where levels under 25-30 nmol/L are considered deficient and indicate increased risk of nutritional rickets or osteomalacia (60). In Sweden, the public guidelines of the National Food Administration recommend maintaining a vitamin D level > 50 nmol/L and defines a level of less than 50 nmol/L as below optimal levels (63). Adding to the complexity of the problem, there are also recommendations from non-governmental medical sources, such as the Endocrine Society (64), that conflict with the previously mentioned national guidelines. The Endocrine Society suggests that keeping a vitamin D level constantly above 75 nmol/L will provide the potential extra-skeletal benefits associated with vitamin D (26). Indeed, there are also studies implying a range between 100–150 nmol/L to be the most appropriate vitamin D level for maintaining optimal skeletal and extra-skeletal health (64-69). However, despite controverses regarding different vitamin D thresholds, the most frequently used cutoffs are <25 nmol/L = severe deficiency, <50 nmol/l = deficiency, 50-75 nmol/L

= suboptimal/insufficient levels, and >75 nmol/L = sufficient levels. Therefore, these are the threshold levels used in this thesis (see Table 1). It is important, though, to point out that the exact threshold for optimal and safe vitamin D levels for skeletal and extra-skeletal health has not yet been established, and it is plausible that these thresholds would vary based on, for example, genetic variance, ethnicity, comorbidities, sex, age, and other factors (70).

It is nearly impossible to obtain sufficient vitamin D levels from dietary sources if one is not eating fatty fish in substantial amounts. The main source of vitamin D in the body comes from UVB exposure, as previously pointed out, which means

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largely unprotected sun exposure. Although this exposure needs to be balanced with the increased risk of skin cancer, sensible sun exposure, together with the use of supplements, is the best way to obtain adequate vitamin D levels (59). A sensible UVB radiation exposure is dependent on the time of the day, latitude, season, and skin pigmentation; however, roughly, exposure of uncovered arms and legs for 5–

30 minutes in the middle of the day, twice a week, is often sufficient (66). In the clinic, there are worries about possible vitamin D intoxication when using vitamin D supplements, since it can cause life-threatening hypercalcemia. Toxic levels of vitamin D are considered to be above 375 nmol/L. Though extremely rare, there are cases of vitamin D intoxication after taking vitamin D supplements. These have occurred due to manufacturing errors, sometimes up to 4,000 times the labeled concentration in supplements, or inadequate prescription by physicians (or inadequate self-administrated treatment), with doses that highly exceed the suggested recommendations (71). The upper tolerable limit of supplement doses, when administrated for a longer period, is 10,000 IU/day, according to several studies (72, 73). However, this upper limit is debated by experts, and Taylor et al.

suggested the upper tolerable limit to be 4.000 IU/day to avoid toxicity (71).

Figure 3. Risk factors of low vitamin D status

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Figure 4. A schematic representation of vitamin D serum level interpretations in different agencies and countries.

Color code: red = severe deficiency (danger), must be corrected without exception; orange = mild deficiency (modest concern), intervention is desirable; green = sufficient supply, no beneficial effects from additional supplementation.

SACN, Scientific Advisory Committee on Nutrition; IOM, Institute of Medicine; DACH, Deutschland (Germany, Austria, and Confederation Helvetica (Switzerland); AAP, American Academy of Pediatrics; IOF, International Osteoporosis Foundation; AGS, American Geriatrics Society. © CT Sempos and N Binkley, 2020 (65).

Table 1.

The most used classifications of vitamin D status, which are also used in the thesis.

Vitamin D concentration a) Classification

≤ 25 nmol/L

≤ 50 nmol/L 51 – 74 nmol/L

≥ 75 nmol/L

> 375 nmol/L

Severe deficiency Deficiency

Suboptimal/insufficiency Sufficient

Toxic To convert nmol/L to ng/mL, divide nmol/L by 2,496.

Extra-skeletal effects of vitamin D In general

As is well known, the main function of active vitamin D is to maintain tight calcium and phosphorus homeostasis in the circulation, which is vital for normal cellular physiology and skeletal health. However, over the last few decades, it has become apparent that vitamin D also exerts profound modulatory effects throughout the body that go beyond calcium regulation (74). The knowledge of vitamin D and its extra-skeletal effects has grown rather exponentially during the last decades, and we now know that vitamin D is also crucial for maintaining optimal somatic health.

Convincing evidence from epidemiological, genetic, in vitro, case-control studies, prospective, retrospective studies, and meta-analyses suggest a link between vitamin D and reduced incidence/morbidity/mortality in diverse somatic illnesses, such as autoimmune diseases (75, 76), cardiovascular diseases (77), certain forms of cancer (78, 79), diabetes (76), allergic diseases, and decreased susceptibility to infectious

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diseases (80). For example, extensive research has shown associations between vitamin D and respiratory tract diseases (81-83), where the most up-to-date research also suggests a relationship between vitamin D and SARS-CoV-2 (COVID-19) infection outcome (84). In most cases, the proposed relationship between vitamin D and different somatic illnesses is based on the immunoregulatory properties of vitamin D, which will be discussed in the next section (74, 85-88).

Of importance for this thesis, vitamin D is also suggested to have a significant impact on normal brain homeostasis (89). Abundant cells and areas in the brain express both VDR and the vitamin D-activating enzyme; thus, a local supply of activated vitamin D in the central nervous system (CNS) is possible, affecting cells in autocrine and paracrine ways (90). Many of these areas have previously been suggested to be involved in the pathophysiology of depression, e.g., the hippocampus, prefrontal cortex, substantia nigra, cingulate gyrus, and hypothalamus. Genetic, animal, and in vitro studies have shown, for example, both regulatory genomic and non-genomic effects of vitamin D in the brain, where vitamin D may have an impact on brain development, function, and maintenance (90-92). Both vitamin D and active vitamin D are able to cross the blood-brain barrier, and there are many ways by which vitamin D may influence, for example, the development and/or severeness of psychiatric illness, e.g., via its effects on catecholamine and serotonin biosynthesis and neurotransmission (93, 94), via its neuroprotective properties (90, 92) and, of highest interest for this thesis, via its immune-modulating effects within the brain as well as on a systemic level (95-97).

The effects of vitamin D on the immune system

First, it must be said that the immune system is an intricate system of interacting signaling and responses on many levels, and sometimes the same inflammatory marker can have seemingly opposite functions in the body. Therefore, the contribution of vitamin D to this complex system cannot be fully described within the framework of the thesis. Consequently, the descriptions herein will be provided to the reader in a simplified form and will focus on the most relevant mechanisms related to the studies included in the thesis.

Active vitamin D shows profound immunomodulatory effects on both innate and adaptive immune responses. VDRs are extensively expressed in key cells of the adaptive immune response, i.e., macrophages, dendritic cells (DCs), and activated T and B lymphocytes. Active vitamin D regulates the proliferation and function of macrophages and DCs, and since macrophages and DCs also possess the vitamin D- synthesizing enzyme, and the vitamin D-activating enzyme, it is possible for vitamin D to act locally in an immunological environment (10, 12). VDRs and the vitamin D-activating enzyme are also largely present in numerous epithelial cells and, thereby, together with cells of the adaptive immune response, vitamin D regulates, and enhances, the host defense via the innate immune response (50).

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The immunomodulatory properties of vitamin D within the adaptive immune response can be briefly described as anti-inflammatory. Vitamin D inhibits the maturation of dendritic cells, which are important for antigen presentation, reduces T cell proliferation, and promotes a shift of T cell differentiation from the pro- inflammatory T-helper 1 (Th1) and Th17 cell response toward a T-helper 2 (Th2) and regulatory T cells (Tregs) cell response, the latter considered to alleviate a pro- inflammatory state in the body and, additionally, to be an important factor in autoimmune diseases (see Figure 5). These potent anti-inflammatory effects of vitamin D have been confirmed in several experimental in vitro and animal studies, and its immunosuppressive effects may prevent the harmful consequences of a prolonged inflammatory response (10, 48, 86, 95, 98).

Active vitamin D exerts its immunomodulatory effects primarily by regulating the expression of cytokines via its interaction with cells of the innate and adaptive immune systems. Cytokines include interferons (IFN), interleukins (ILs), chemokines, lymphokines, and tumor necrosis factor (TNF), and they are important both in health and disease. The pro-inflammatory Th1 and Th17 immune response mediates the production of cytokines, such as IFN-γ, TNF-α, IL-21, IL-6, IL-8, IL- 12, IL-1β, IL-2, IL-23, and IL-17. As previously noted, the Th1 response is inhibited by vitamin D, thereby leading to lower levels of these pro-inflammatory cytokines.

Also, vitamin D increases the Th2 cytokines IL-10, IL-4, and IL-5, with a subsequent enhancement of Th2 cell differentiation, which in turn increases the production of Treg cytokines. Treg-related cytokines are, among other immunological biomarkers, such as transforming growth factor-beta (TGF-β) and IL-10, suggested to stimulate a reduction in pro-inflammatory cytokines (10, 11, 80, 86, 98). Although promising results exist from experimental in vitro and animal studies, the immunomodulatory effects of vitamin D have been hard to demonstrate in humans, and few studies have confirmed the suppressive effects of vitamin D on Th1 cell response (80, 90).

One of vitamin D’s critical immunomodulatory effects in the brain originates from microglia, the primary immune effector cell in the CNS, which interestingly comprises the vitamin D-activating enzyme. Rapid activation and response by different immune cells during infection, injury, or disease is crucial to protect the brain and bring the CNS environment back to a healthy condition. A prolonged inflammatory state—that is, chronic inflammation—could be detrimental to the CNS and has been associated with several neurological diseases (99). Notably, in animal studies, activated microglia have been shown to increase their expression of VDR and the vitamin D-activating enzyme upon an immunological challenge, with a subsequent enhancement of their receptiveness to vitamin D. Several studies have shown that activated microglia exposed to vitamin D and active vitamin D reduce their expression of pro-inflammatory cytokines and increase their expression of IL- 10 (100). Thus, activated microglia have the possibility of promoting the previously mentioned switch from a harmful Th1 cell response to a less inflammatory Th2 cell

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response. Altering the immune response via microglia suggests that vitamin D is a control mechanism for avoiding a prolonged inflammatory state in the CNS (101).

In my thesis, I investigated the relationship between vitamin D and the following inflammatory markers: TNF-α, IL-6, IL-1β, IL-2, IL-13, IL-8, IL-10, IFN-γ, and C- reactive protein (CRP). Also, in one of the included studies, we explored the relationship between vitamin D and the neutrophil-to-lymphocyte ratio (NLR) and white blood cell count (WBC), two markers considered to be inflammatory markers of systemic, low-grade inflammation (102).

Figure 5. Effects of Vitamin D on the immune system.

Mechanism 1: Vitamin D promotes a shift of T cell differentiation from the pro-inflammatory Th1 and Th17 cell response toward a Th2 and Treg cell response, the latter considered to alleviate a pro-inflammatory state in the body.

Mechanism 2: Vitamin D inhibits the maturation of dendritic cells (DCs).

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Vitamin D in MDD and suicidality

Vitamin D in depression

Research on vitamin D and depression has increased over the last 10–15 years. Most studies on vitamin D and psychiatric illness have used cross-sectional study designs;

however, support for a relationship also comes from several systematic reviews, meta-analyses, and clinical trials. The main part of these studies relates to the association between vitamin D and MDD, psychotic disorders (predominately schizophrenia) (103, 104), neuropsychiatric disorders (attention deficit hyperactivity disorder [ADHD], autism) (104-106), and dementia (104). In cases of disorders strongly connected to neurocognitive function, animal studies have also frequently been used. In brief, the psychiatric disorders mentioned above are suggested to have some association with vitamin D, mainly via its impact on important brain functions and areas that subsequently lead to impaired neurocognition, such as memory functioning, executive function, processing speed, and attention, to name a few. Additionally, some studies imply that low prenatal levels of vitamin D increase the risk of developing schizophrenia and the cognitive impairments seen in ADHD.

Regarding vitamin D’s relationship with MDD, the majority of previous studies, systematic reviews, and meta-analyses included show an association between low levels of vitamin D and depression (107-109), although there are also studies showing opposite results (110, 111), which might be due to methodological differences between studies (112). Some studies suggest that vitamin D is specifically associated with more severe depressive symptoms (i.e., associated with increased depression rating test scores) (113, 114), as in, for example, a large cohort study, the Netherlands Study of Depression and Anxiety (NESDA) (114). They investigated the association between vitamin D and depressive disorders (major depressive disorder and dysthymia), as well as the association between vitamin D and clinical depressive characteristics. In this study, low levels of vitamin D were associated with both the presence and severity of depressive disorders, suggesting that low levels of vitamin D might constitute an underlying biological vulnerability to depression. A special type of depressive disorder is seasonal affective disorder (SAD), which is characterized by symptoms of depression, anxiety, irritability, appetite changes, hypersomnia, and fatigue that occur during the winter months and diminish in the spring and summer. The incidence of SAD has been shown to increase with higher latitude (reduced sun exposure); therefore, SAD has also been suggested to be related to vitamin D (115). Also, there are studies suggesting that low levels of vitamin D may contribute to psychopathology only in certain types of depression, such as “suicidal depression” (116, 117) or “inflammatory depression”

(117-120), and also that low vitamin D is relevant only at certain levels of deficiency (121).

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Although there are promising results from epidemiological studies linking low vitamin D to depression, the possible benefits of vitamin D treatment for depression have not yet been demonstrated. There is presently a lack of well-designed randomized controlled trials (RCTs) (122), and it is therefore impossible to draw any conclusions regarding the potential causal effects of low vitamin D in relation to depression. Some RCTs have shown positive effects of vitamin D treatment on depressive symptoms, or the risk of developing depression over time (123, 124), however, overall, there is not yet sufficient evidence to recommend vitamin D supplements to treat or prevent depression (107, 125). In one recent, well-designed RCT, in which MDD patients with concurrent severe vitamin D deficiency were treated with vitamin D/placebo in addition to antidepressant treatment (escitalopram), the authors found that vitamin D levels did not correlate with subjective or objective depression ratings (126). Nonetheless, the patients who received placebo, and thus had uncorrected vitamin D levels, required significantly higher doses of escitalopram (mean difference, 4 mg/day) than those whose vitamin D levels were corrected. To the author’s surprise, 25% in the placebo group had increased their levels from severely deficient to sufficient levels at the end of treatment, suggesting an intake of vitamin D supplements. In contrast, about 30%

in the intervention group did not reach sufficient vitamin D levels at the end of treatment, suggesting that some of the patients did not take their subscribed capsules. Therefore, a post hoc analysis was conducted among study completers, showing that end-point escitalopram levels were significantly higher (by 4 mg/day) among patients with vitamin D deficiency compared to those with sufficient levels.

Vitamin D in suicidality

The research on vitamin D’s relationship with suicidality has for a long time been limited. However, several studies have been published on the subject during the last three years, of which three are included in this thesis. (116-118, 127-134). Three recent studies have confirmed an association between vitamin D deficiency and increased suicide risk and/ suicidal ideation. One prospective study by Fond et al.

(2019) investigated the possible association between severe vitamin D deficiency (<25 nmol/L) and suicide risk, along with other factors related to psychiatric illness, such as anxiety, psychosis, and depressive symptoms (129). The study sample comprised 251 schizophrenic outpatients, of which approximately 30% were severely deficient in vitamin D. They found that severe vitamin D deficiency was associated with suicide risk and several other factors, such as negative symptoms and antidepressant consumption. In another study, Gokalp et al. (2020) showed that adolescents patients (n=215), admitted to a pediatric hospital after a suicide attempt had significantly lower vitamin D levels than healthy individuals (n=200) (30 nmol/L and 50 nmol/L respectively) (128). Lastly, Kim et al. investigated whether the risk of having suicidal ideation was associated with vitamin D levels in healthy adults (n=157.211) (116). Using the vitamin D cut-off levels of <25 nmol/L to

Vitamin D in Depression and Suicidality Grudet, Cécile

Vitamin D in Depression and Suicidality

Vitamin D in Depression

and Suicidality

Vitamin D in Depression and Suicidality

Vitamin D in Depression and Suicidality

I dedicate this thesis to all people who face the challenges of long-lasting depressive symptoms in their lives.

You are strong and brave and deserve the best of support

anyone can get!

Table of Contents

Abstract

List of papers

Preface

Populärvetenskaplig sammanfattning

Bakgrund

Material och metoder

Resultat

Slutsatser

Abbreviations

Definitions

Vitamin D

Psychiatry

Context of this thesis

Introduction

Major depressive disorder and suicidality

An introduction to vitamin D

Vitamin D in MDD and suicidality